Project Summary Dilated cardiomyopathies, heart failure (HF), and arrhythmias are a significant health burden. Despite the improving medical therapies, cardiac resynchronization therapies, and left ventricular assist devices, HF remains a global epidemic. The lifetime risk of developing HF is 20% with a 5 year age-adjusted mortality at 59% and 45% for men and women, respectively. Thus, the identification of pathways underlying development and progression of HF and arrhythmias is essential for the creation of improved diagnostics and treatments. Over the past two decades, the cardiac cytoskeleton has emerged as a central governing factor in the control of cardiac membrane integrity, and dysfunction in cytoskeleton and cytoskeletal-associated proteins has been directly linked with a host of human cardiac pathologies, most notably cardiac myopathies and dystrophies. In fact, human loss-of-function variants in cardiac cytoskeletal or cytoskeletal-associated genes that alter myocyte signal transduction, myocardial mechanics, and force transmission are now directly linked with dilated cardiomyopathy, muscular dystrophy, and arrhythmogenic cardiomyopathy. In contrast to myopathy and dystrophy fields, the role of the cytoskeleton in normal electrical function is not well resolved. Further, until only recently, human arrhythmia mechanisms were limited to mutations in ion channels. However, our group and now others have defined a second class of arrhythmias due to mutations in channel-associated proteins. Dysfunction in these proteins is linked with diverse pathologies including defects in channel synthesis and targeting, gating, and post-translational modifications. While this information has been important for new disease diagnosis and fundamental cardiac cell biology, there remain large cohorts of phenotype positive/genotype negative patients with familial forms of HF and arrhythmia. Further, there remain large knowledge gaps regarding the pathways underlying more common forms of acquired HF and arrhythmia. The overall goal of my program is to define new cell and molecular pathways underlying HF and arrhythmia. Based on clinical and genetic findings, we uncovered a new and essential cytoskeletal-based pathway critical for cardiac electrical function. Our preliminary data, that spans human to molecule, utilizes new in vivo mouse models and targeting strategies and innovative technologies supports our central hypothesis that the cytoskeletal protein ?II spectrin serves as an unexpected and integral regulatory node for the organization of critical myocyte membrane and membrane-associated proteins. Further, our data support that dysfunction in this pathway is an underlying factor for cardiac electrical and structural remodeling in HF and arrhythmia. Our proposal will test the new roles of the ?II spectrin pathway in HF and arrhythmia as well as the molecular mechanisms underlying ?II spectrin regulation in disease: We will 1) Define the in vivo role of ?II spectrin in heart failure; 2) Define the cell and molecular roles of ?II spectrin in heart failure; 3) Define new in vivo molecular mechanisms underlying cardiac ?II spectrin regulation.